Technical Considerations for Co-Locating UWB and GPS Radios
Tyler H. Van Slyke, Kansas State University, 2009
Measuring Interference from a UWB Transmitter in the GPS L1 Band
Tyler Van Slyke, William B.Kuhn, Balasubramaniam Natarajan
IEEE Radio and Wireless Symposium, pp. 887-890, January 2008
GPS and UWB
In 2002, the FCC designated part of the frequency spectrum, from 3.1 GHz to 10.6 GHz, as Ultra-Wide Band (UWB). Specifically, the FCC defines UWB as any antenna whose transmission bandwidth is 500 MHz, or whose fractional bandwidth is at least 20%. The WiMedia standardization of the UWB, popular within the communications industry, is composed of 14 sub-bands of 528 MHz each, placed into 5 groups. An illustration of the UWB and its WiMedia divisions can be seen below:
A major use of this band so far has been wireless data transmissions between a central hub and various media devices, such as cameras, flash drives, printers, etc. Popular consumer electronics, such as the aforementioned devices and others, are beginning to include this technology more commonly, and use of such devices may grow as the WiMedia scheme becomes cheaper and easier to implement. This is because UWB devices (that use MB-OFDM) bear many advantages over currently-used designs, such as:
Higher levels of security
Higher data rates
Highly integrable - may be used with a wide variety of electronic devices
Resistance to jamming, interference, and detection due to low transmit power
Problems with GPS
The GPS L1 band, used by consumer GPS devices, is designated at 1.57542 GHz, and typical GPS devices use a signal bandwidth of 1 MHz. The minimum signal power by which a GPS device in ideal conditions can receive is -130 dBm, though thermal noise can degrade this by more than 15 dB. In consideration for this signal band, the FCC placed a limit on UWB devices of -75.3 dBm / 1 MHz of signal power at 1.57542 GHz, in order to prevent interference with GPS devices. However, one publication documents how, even with this restriction, UWB devices can still significantly curb the receive-sensitivity of GPS devices, hindering their performance.
Theoretical analysis in the above publication calculated that, in room-temperature conditions, a UWB device (emitting -75.3 dBm/1MHz power at 1.57542 GHz) placed at least 1.33 meters away from a receiving GPS antenna will double the noise floor of the GPS device, which then degrades signal reception by 3 dB.
In a research effort supported by Garmin, Inc. engineers at K-State were able to characterize the real-world effects of co-located GPS and UWB devices, the first known experiment of its kind.
Tests and Measurements
The following image is a diagram of the testing environment:
The idea was to setup a UWB device (a UWB hub and dongle, here) alongside a GPS antenna, so that the amount of signal power transmitted to the GPS device could be measured. The UWB device used is shown here:
Initially, the GPS antenna was placed 1 cm away from the UWB device's antenna, as pictured below:
It was found that in this configuration the interference was different depending on the activity of the UWB device. If the UWB device was sitting idly, the interference picked up by the GPS antenna was insignificant. However, when the UWB device was engaged (e.g. transferring a file from a computer to the UWB dongle), interference occurred. A screenshot of the spectrum analyzer used to capture this data can be seen here:
What was found was that the noise floor of the GPS antenna rose by about 1 dB when the UWB device was transferring files. The spikes throughout the spectrum shown are believed to be due to the frequency-hopping of nearby UWB devices. Despite this increase in noise power, the antenna of the UWB device wasn't actually the most problematic component of the UWB implementation.
As it turned out, radiation power from the UWB chipset itself was a much larger contributer to noiser power when co-located with a GPS antenna. Measurements were taken of the received power on the GPS antenna when placed 1 cm away from the actual UWB device's circuitry, which can be seen here:
This configuration led to much more devestating results for the GPS antenna. Below is a screenshot of the spectrum analyzer when the GPS antenna was placed 1 cm away from the UWB hub's chipset:
In this photo the noise floor has risen by 7 dB! But that isn't the worst-case scenario. When the GPS antenna was placed 1 cm above the central processor of the UWB device, it was found that the noise floor of the GPS antenna was raised by 10 dB.
As a result of this research, it was found that a UWB hub's antenna wasn't main problem regarding interference with GPS devices. Instead, it was the radiation coming from the UWB device's chipset itself that would prevent co-location with a GPS device. Below is a composite of images that show how the UWB device interfered with the GPS device under various modes:
Lastly, these K-State researchers speculated that with the use of strategic component-placement on boards, as well as circuitry-shielding for UWB devices, co-location with GPS systems could be possible. As a first work of its kind, more literature is needed on this subject, since there is a fighting chance that UWB devices could one day be as popular as cell phones.